1. Field of the Invention
The present invention relates to a load control system comprising a plurality of load control devices for controlling the amount of power delivered to a plurality of electrical loads from an AC power source, and more particularly, to a lighting control system for controlling the intensity of a plurality of lighting loads.
2. Description of the Related Art
Typical load control systems are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. A load control system generally comprises a plurality of control devices coupled to a communication link to allow for communication between the control devices. The control devices of a lighting control system include load control devices operable to control the amount of power delivered to the loads in response to digital messages received across the communication link or local inputs, such as user actuations of a button. Further, the control devices of a lighting control system often include one or more keypad controllers that transmit commands across the communication link in order to control the loads coupled to the load control devices. An example of a lighting control system is described in greater detail in commonly-assigned U.S. Pat. No. 6,803,728, issued Oct. 12, 2004, entitled SYSTEM FOR CONTROL OF DEVICES, which is incorporated herein by reference in its entirety.
FIG. 1 is a simplified block diagram of a prior art lighting control system 10 according to the present invention. The lighting control system comprises a power panel 12 having a plurality of load control modules (LCMs) 14 (i.e., load control devices). Each load control module 14 is coupled to a lighting load 16 (or another type of electrical load, such as a motor load) for control of the amount of power delivered to the lighting load. Alternatively, each load control module 14 may be coupled to more than one lighting load 16, for example, four lighting loads, for individual control of the amount of power delivered to each of the lighting loads. The power panel 12 also comprises a module interface (MI) 18, which controls the operation of the load control modules 14 via digital signals transmitted across a power module control link 20
The lighting control system 10 further comprises a processor 22, which controls the operation of the lighting control system and thus the amount of power delivered to the lighting loads 16 by the load control modules 14. The processor 22 is operable to communicate with the module interface 18 of the power panel 12 via a power panel link 24. Accordingly, the module interface 18 is operable to cause the load control modules 14 to turn off and on and to control the intensity of the lighting loads 16 in response to digital messages received from the processor 22. The processor 22 is operable to be coupled to a plurality of power panels (not shown) via the power panel link 24.
In addition to being coupled to the power panel link 24, the central processor 22 is also coupled to a control device communication link 26 for communication with a plurality of control devices 28 (e.g., wallstations or keypads). The control devices 28 allow users to provide inputs to the lighting control system 10. The processor 22 is operable to control the lighting loads 16 in response to digital messages received from the control devices 28.
The control devices 28 of the control device communication link 26 communicate using a high baud rate, e.g., 125 kilobits per second (kbps), and are wired together using a daisy-chain wiring scheme. Using the daisy-chain wiring scheme, the control devices are wired in series, e.g., a first control device is wired to a-second control device, which is wired to a third control device, which is wired to a fourth control device, and so on. The control devices cannot be wired using a web, star, or “free-wiring” topology. Since the control device communication link 26 uses a high baud rate of 125 kbps and a daisy-chain wiring scheme, the length of the link is limited to approximately 2000 feet.
The length of the control device communication link 26 may be effectively lengthened by using a plurality of repeater devices 30. The plurality of repeater devices are coupled between different sections of the control device communication link 26, which are each limited to 2000 feet. Each repeater device 30 receives the AC line voltage and supplies power for the control devices on one of the sections of the control device communication link 26. The repeater devices 30 are operable to retransmit the digital messages that are received on one section of the control device communication link 26 on the other section of the link to which the repeater devices are connected.
The use of the repeater devices 30 introduces some delay into the transmissions of the control device communication link 26. When a repeater device 30 retransmits a digital message, there is a delay period from when the repeater device 30 receives the digital message to when the repeater device transmits the digital message on the other section of the control device communication link 26. Further, depending upon the data content of the digital message, the repeater device 30 may be enabled to transmit on the control device communication link 26 for a period of time after the end of the digital message that the repeater device. Thus, there is a period of time after the repeater device transmits a digital message that the repeater device 30 maintains control over the communication link 26 and the other control devices cannot transmit digital messages.
Accordingly, a predetermined delay period must be built into the protocol of the control device communication link in order to account for the delays of the repeater devices 30. Specifically, each control device must wait for a predetermined amount of time after the end of the last digital message before transmitting a digital message on the communication link. The predetermined delay period is dependent upon the number of repeater devices 30 that can be included in the lighting control system 10. The predetermined delay period decreases the response time of the lighting control system 10.
Thus, there is a need for a load control system that can include a plurality of repeater devices, but still has a substantially fast response time.